Ethylene-propylene polymers have long been known, and used for a variety of applications. For example, such copolymers, frequently referred to in the art as olefin copolymers or OCPs, have long been recognized as viscosity index improvers in engine lubricating oils. In addition to that application, such polymers have also been widely used as impact modifiers for plastic compositions.
As used in lubricating oil compositions, ethylene-propylene polymers have the ability to provide a high thickening contribution to the lubricating oil composition with which the OCP is mixed to provide increases in the viscosity index of the overall composition. Thickening power is often defined as the difference in the kinematic viscosities of a dilute solution of an OCP mixed with oil and the diluent oil. For example, an oil solution containing from 1 to 2 percent of an OCP which provides a thickening power of 6 to 7 centistokes measured at 100.degree. C. generally indicates acceptable viscosity index improvement performance.
For a given class of polymers, the higher the molecular weight, the higher is the viscosity of a lubricating oil containing the OCP. However, higher molecular weight polymers exhibit a greater tendency to break down under the shear and high temperature conditions normally found in engine operation, frequently resulting in the loss of viscosity. Accordingly, viscosity index improvement often depends on the balance between the thickening contribution of the OCP and its tendency to degrade, referred to as shear stability. Shear stability is typically defined as a percent viscosity breakdown on shear under a standard set of conditions. A value below 30 percent viscosity breakdown in an OCP is generally an indication that the viscosity index improver OCP is shear stable as that term is understood in the art.
Another important characteristic required for a viscosity index improver is viscosity at low temperatures, which relates to the ease of engine cranking during start-up in cold climates. An ideal viscosity index improver exhibits a negligible viscosity contribution at low temperatures while providing a large viscosity contribution at engine operating temperatures. At the same time, an ideal viscosity index improver exhibits a low tendency to degrade, and consequently a high shear stability under engine operating conditions.
Accordingly, in formulating lubricating oils to satisfy the varying conditions desired, it has generally been the practice to select those polymers which provide at the lowest cost the best overall balance of properties including viscosity at performance temperatures, shear stability and low temperature viscosity.
In the past, the art has employed as viscosity index improvers, solid amorphous ethylene-propylene polymers. When selecting ethylene-propylene polymers, a molecular weight was chosen so that the polymer would provide shear stable viscosity index improvement after the oil dispersion process. These solid ethylene-propylene polymers were generally sheared during the oil dispersion, solution process resulting in lower molecular weight ethylene-propylene polymers. Oil concentrates of these lower molecular weight ethylene-propylene polymers were utilized as viscosity index improver packages by oil formulators. The sheared ethylene-propylene polymers that dissolved in oil, if isolated or prepared neat, were no longer stable solids but would exhibit extreme cold flow properties or exist as viscous oils. Such ethylene-propylene polymer viscosity index improvers as used in the prior art typically exhibit a Reduced Solution Viscosity or RSV, measured on a 0.05 weight % solution of the polymer in declain at 135.degree. C., less than about 1.5 dl/g when they provide shear stable viscosity index improvement. Near that RSV limit, such ethylene-propylene polymers are difficult to recover and package using conventional rubber coagulation, recovery and packaging equipment. Those handling problems are even more severe as the RSV decreases, making recovery of such ethylene-propylene polymers at RSVs below 1.4 dl/g virtually impossible because the ethylene-propylene polymers show excessive cold flow or are essentially in the liquid phase.
Thus, the art has found that the use of ethylene-propylene polymers at RSVs of less than 1.5 dl/g require more elaborate techniques for the recovery of the polymers. In addition, such polymers may be sticky and therefore require special packaging for suitable containment.
Past efforts to handle such polymers as concentrated solutions in oil containing 5 to 20 percent by weight of the ethylene-propylene polymer viscosity index improver have met with limited success by reason of the disadvantages of requiring large amounts of oil in which the viscosity index improvers must be dispersed, and that in turn results in additional costs in transportation and, in some cases, additional tariffs by reason of the oil present even though the polymer is the component of interest.
The prior art has recognized the need to improve low temperature performance of such viscosity index improvers. For example, U.S. Pat. No. 4,507,515 describes blends of polymer compositions in which the major component has a low ethylene content and the minor component has a higher ethylene content, the minor component generally containing less than 10 percent by weight based on the weight of the blend. Similarly, U.S. Pat. No. 3,697,429 likewise describes the use of blends of high and low ethylene content polymers to achieve improvement in low temperature properties of a lubricating oil composition. One of the shortcomings of both prior art patents is that they do not describe any technique by which the major and minor components can be handled as solids.
In Canadian Patent No. 911792, there is disclosed a process for shearing high molecular weight polymers to produce a polymer having a desired molecular weight without altering the molecular weight distribution for use as a viscosity index improver. That Canadian patent does not, however, address the need for a solid shear stable polymer formed by shearing the polymer to reduce its molecular weight and its molecular weight distribution.
The present invention addresses the need to provide an oil-free or solid polymer having the capability of functioning as a shear stable viscosity index improver. According to the concepts of the invention, the ethylene-propylene polymer of the invention is the product of a simultaneously blended and sheared blend containing a high ethylene content ethylene-propylene polymer and a lower ethylene content ethylene-propylene polymer whereby the physical properties of the sheared blend allows it to be recovered by means of conventional plastics processing equipment. The composition of the present invention in which the two components are simultaneously blended and sheared can thus be used as a viscosity index improver exhibiting improved low temperature properties as compared to (a) either of the two components used in preparing the blend and (b) blends of the same two components prepared by other blending techniques. The blends of the present invention find use not only as viscosity index improvers, but can also be used to improve the impact strength of various plastics such as polyamides or nylon, polyesters, polyolefins and like thermoplastic and thermosetting compositions, or as antiozonants for rubber/rubber blends.
It is accordingly an object of the present invention to provide a solid ethylene-propylene polymer blend which overcomes the shortcomings of the prior art.
It is a more specific object of the invention to provide an ethylene-propylene polymer blend and a method for its preparation which can be used to improve the viscosity index of lubricating oil compositions as well as an impact modifier for plastic compositions in which the blend is subjected to simultaneous blending and shearing to reduce the molecular weight and the molecular weight distribution of each of the ethylene-propylene polymer components.
It is furthermore an object of the present invention to provide a solid ethylene-propylene polymer blend which can be prepared in conventional plastic processing equipment.
These and other objects and advantages of the present invention will appear more fully by way of the following description.